1. Field of the Invention
The present invention relates to an optical network, in particular to an optical fiber for use as a transmission line in such an optical metro-area network.
2. Description of the Related Art
In general, optical fibers, used in metro networks, have a negative dispersion characteristic. The fibers include core with a high refractive index and a clad surrounding the core. An annular region having a refractive index lower than that of the core may be interposed between the core and the clad.
U.S. Pat. No. 4,715,679 to Bhagavatula, which is entitled “Low Dispersion, Low-Loss Single-Mode Optical Waveguide,” discloses a single mode optical waveguide consisting of a core having an annular refractive index depressed region and a clad enclosing the core.
When configuring a metro network, it is more economical, in a lower transmission rate, to configure the metro network in a direct modulation (DM) mode than in an external modulation (EM) mode.
FIG. 1a is a schematic diagram illustrating a direct modulation mode and FIG. 1b is a schematic diagram illustrating an external modulation mode.
Referring to FIG. 1a, light output from a laser diode (LD) 110 is modulated by applying a direct current (DC) voltage IDC and data to the laser diode (LD) 10. It is known that a directly modulated optical signal exhibits a positive chirp characteristic.
Referring to FIG. 1b, light is produced by applying DC voltage IDC to a laser diode 210, and a modulator 220 receives and modulates the light to the input data.
A conventional metro network or access network is configured using standard single mode fibers (SSMFs). In consideration of the fact that a directly modulated optical signal exhibits a positive chirp characteristic, a conventional method for configuring a metro network using non-zero dispersion shifted fibers (NZDSFs) having negative dispersion values (about −7 to −8 ps/nm/km at 1550 nm) is also known. However, such a method needs many restrictive requirements in order to achieve an effect on transmission characteristics. Furthermore, it is noted that when an optical communication network uses a 10 Gbps transmission rate and a 1550 nm band direct modulation mode, the SSMFs can be applied to a transmission distance of about 10 km and NZDSFs having negative dispersion values can be applied to a transmission distance of about 75 km. The SSMFs have difficulty providing good transmission characteristics due to the chirp phenomenon caused by the direct modulation, and the NZDSFs are not effective in a network longer than 100 km due to the excessively high negative dispersion values thereof. When a metro network is configured using SMFs, configuring such a network become more complex because one or more separate dispersion control optical fibers must be used.
FIG. 2 is a graph showing Q-factor curves with respect to individual transmission distances for a typical SMF and negative dispersion fibers (NDFs). FIG. 2 shows Q-factor curves of an optical signal with preamp and with 5 dB extinction ratio proceeds through a first NDF, in which an optical signal with preamp and with 8 dB extinction ratio proceeds through a second NDF, in which an optical signal without preamp and with 8 dB extinction ratio proceeds through a third NDF, and in which an optical signal without preamp and with 8 dB extinction ratio proceeds through an SMF, respectively. It can be seen that the typical SMF has difficulty providing good transmission characteristics due to the chirp phenomenon caused by the direct modulation, and the typical NDFs have a restriction in transmission distance in a metro network due to the high negative dispersion values thereof.
FIG. 3 is a graph illustrating characteristics of an erbium doped fiber amplifier. FIG. 3 shows gain curves in the individual cases in which a −40 dBm optical signal is input, in which a −10 dBm optical signal is input, and in which a +5 dBm optical signal is input; and a noise figure curve for a −10 dBm optical signal. The channel efficiency decreases in a wavelength band in the range of 1560 to 1570 nm, which is the dead zone of the erbium doped fiber amplifier.
It is also noted that because a typical NZDSF has dispersion values suitable for C-band (1530 to 1565 nm), it has a restriction in using L-band and contributes to deteriorate the channel efficiency of a metro network in combination with the dead zone of the erbium doped fiber amplifier.
FIG. 4 is a graph showing a refractive index profile of a typical NZDSF having negative dispersion values. The NZDSF includes a double-ring shaped core located at the center of the NZDSF, a refractive index depressed region, and a clad. The NZDSF has a poor coupling efficiency with an existing optical fiber due to its complicated refractive index profile. In addition, the NZDSF has a problem in that macro bending loss is very high due to its large refractive index depressed region.
Accordingly, there is a need in the art for an optical fiber having optical characteristics suitable for a 2.5 Gbps transmission rate, which is a principle transmission rate of metro networks at present, and a 10 Gbps transmission rate which will be widely used in the future.